In known laser beam welding devices, a laser beam generated by a laser beam source is focused on the joining zone between two workpieces. The cross section of the focused laser beam is circular, the focus diameter ranging between 150 μm and 600 μm in conventional applications. The absorption of the laser beam into the material results in heating and melting in the joining zone. The molten material is mixed together as a function of the joint geometry and material properties, resulting in a connection between the joining partners after solidification. To produce a weld, a relative movement is generated between the laser beam or laser welding focus and the tool, i.e., either the laser beam or the workpiece is moved at a defined feed rate. For example, to produce a circumferential weld (radial weld) on the lateral surfaces of two cylindrical components, the components are rotated around their axes of symmetry by a rotating device, while the laser beam source, and therefore the circular laser beam welding focus, remains stationary. The machining time is limited, among other things, by the influence of the centrifugal force, which increases as the machining speed increases, resulting in an unwanted ejection of molten material from the joining zone. A further problem in known laser beam welding devices is the fact that the workpieces must be twisted by more than 360° if the circumferential weld to be produced must be tight, i.e., if the circumferential weld must be closed in the circumferential direction. The welding angle is therefore larger than 360°, which produces a weld overlap zone which is formed due to the fact that the laser beam welding focus passes over this zone a second time. This weld asymmetry causes the component to warp, as a result of which the component does not run true or even bends. This may result in functional impairment or even failure of the component, in particular if the component is a fuel injector.
A laser beam welding device is also described in German Patent Application No. DE 20 2005 015 263 U1 in which a plurality of laser diodes is situated in an annular configuration for the purpose of generating an annular laser beam focus. The disadvantages of this arrangement are the large number of laser diodes required and the fact that it appears to be difficult to achieve a uniform intensity distribution in the region of the laser beam welding focus, making it impossible to dispense with rotating the joining partners or the laser diode arrangement when producing a circumferential weld.